Treatment of metals



of oxidation and other corrosive agencies.

Patented Dec. 1, 1942 UNITED STATES PATENT OFFICE TREATMENT OF METALS Frank B. Quinlan and Lloyd P. Grobel, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York 9 Claims. (01. 117-71) This invention relates to the art of treating metals so as to protect them against the effects The invention more specifically is concerned with the treatment of such metals as iron, steel and cop per, which readily oxidize at elevated temperature. This application is filed in place of our abandoned application Serial No. 181,371, filed December 23, 1937.

It is now well known that oxidizable metals of the kind mentioned above may be very effectively protected by treating the surface with aluminum so as to cause the aluminum to alloy with the foundation metal. The trade name for this treatment of metal is calorizing. This fundamental idea is disclosed in a patent to Tycho Van Aller, No. 1,155,974, dated October 5, 1915. According to the Van Aller process, the article to be treated is brought into contact with aluminum powder at a proper temperature and under conditions which prevent oxidation of the metallic article. The treatment takes place in a closed oven which slowly rotates. After being thus treated for a proper length of time, which may be about two hours, the article is fired at about 700 to 800 found that aluminum powder in suspension in a vehicle formed of a binder and a volatile solvent may be applied to the surface to be protected in the way paint is applied and in cold condition. The coated. article is then fired at a temperature which will cause the aluminum to alloy with the foundation metal. An example of such a process is disclosed in a patent to Goodwin Howe, No. 1,655,269.

Although in a general way the aforementioned processes are quite satisfactory, some of them are more or less limited in their application to articles of certain shapes and sizes or for use under limited conditions. Immersion in molten aluminum is not always practicable, since some metals, such as non-ferrous metals, may be injured or even dissolved in such a bath. Furthermore, the maintenance of a large bath of molten aluminum is expensive. This method is also objectionable in that it does not permit reundesirable, particularly on repair jobs.

pairs to be made to damaged surfaces in place. 65

The repair can be made only by disassembling the structure. This is costly and time-consuming and in many cases, for example with boiler or superheater tubes, may completely disrupt plant operations for a prolonged period. Spraying the powdered metal carried in a vehicle containing an inflammable volatile solvent involves health and fire hazards which may make the method Further, the known methods have all necessitated that the coated article be baked prior to use in order to alloy the aluminum coating with the oxidizable foundation metal. Obviously such a baking step adds materially to the cost of the calorized article. It has been realized for quite some time that a method which would obviate the disadvantages of the described process and result in calorized articles of longer service life at lower unit cost would greatly broaden the field of utility for this kind of metal protection.

It is a principal object of the present invention to provide an improved method of protecting copper, ferrous and other oxidizable metals from oxidation at normal or at elevated temperatures. The preferred embodiment of the invention provides a method which is applicable in forming an initial protective heat-resisting coating on an assembled, installed structure or in repairing or renewing such coating. The same method also may be used in protectively coating unassembled metal parts. The scope of the invention also includes the improved products of the method.

The following description is illustrative of how our invention may be carried into effect.

The oxidizable foundation metal, for instance low carbon or alloy steel, is first thoroughly cleaned in any suitable way to remove all grease, dirt or scale. For example, the article may be cleaned by immersion in a suitable pickling bath such, for instance, as a dilute solution of hydrochloric acid. Cleaning of large objects or of installed metal parts or structures, for example a bank of boiler tubes, advantageously may be done by grit blasting using, for instance, finely divided steel particles.

With minimum delay after cleaning, the metal surface is then coated with a thin film of metallic aluminum. Coating preferably is done by spraying. We prefer to use the so-called Schoop process of spraying metal in depositing the aluminum on the foundation metal. By this method a small diameter aluminum wire is fed into a suitably hot flame, such as an oxyacetylene flame. To obtain reasonably fast spraying aluminum wire, inch in diameter is preferred.

' penetrate to the foundation metal.

We have used soft aluminum wire having a content of metallic aluminum of 99 per cent or more with satisfactory results. The molten metal is then broken up into small drops by an air blast and deposited upon the surface-to be protected. We have found that the spray-deposited unbaked coating is not sufficiently dense to prevent the infiltration of air through the coating to the foundation metal when the coated article is repeatedly or continuously heated at temperatures of the order of 500 to 600 C. As a result the surface of the foundation metal is oxidized and the service life of the article is lessened.

In accordance with one embodiment of the present invention, oxidation of the foundation metal is prevented, and the service life of the article is increased, in the following manner: Over the spray-deposited, unbaked aluminum coating is applied a suitably thick second coating of a metal capable of alloying with aluminum or alloys thereof to form a sealing layer through which air at high temperature will not We prefer to use the Schoop process of metal spraying above described, since this method provides a high ratio of volume to surface area of the individual sprayed particles or globules. As a result, oxide formation is relatively low and the sprayed metal adheres more tenaciously to the underlying surface.

By applying first an aluminum coating and then a coating of a second metal, a composite coating is obtained. After being applied, and particularly during initial service use at an elevated temperature, the aluminum coating alloys at least in part with the foundation metal, and the two metallic coatings alloy at least in part with each other. The resulting coating is practically impervious to air at a high temperature.

As a sealing metal we prefer to use metals or alloys which are solid at normal temperature and have a melting point lower than the melting point of aluminum, that is, below approximately 660 C. Examples of such metals are tin, lead, antimony, bismuth and cadmium. We have obtained best results by using tin, and therefore prefer this metal over others. The sealing coating may be of any suitable thickness. For example, when the aluminum layer deposited on the foundation metal is 6 to 8 mils thick, the sealing layer of metal may be 3 or 4 mils thick.

For some applications of the calorized article a final coating of aluminum applied over the layer of sealing metal may be advantageous. Preferably such external aluminum coating is applied by the Schoop process. However, other methods such as immersion in molten aluminum may be employed, as desired or as conditions may require. Such aluminum coating alloys at least in part with the underlying metallic coating after applying or during initial service use.

Oxidizable foundation metals when protectively metallized in the manner above described are thereby rendered inoxidizable at high temperatures without the necessity for any separate baking step prior to initial use of the material. The composite metallic coating is hard, tough, dense, continuous and adheres tenaciously to the underlying foundation metal. We are unable to state at this time any specific theory in explana tion of our improved results and as to why our method makes it possible to eliminate the conventional baking of an aluminum-coated oxidizable metal. One possible explanation is that alloying of the coating layers to each other and to the foundation metal continues slowly to completion during the high temperature conditions of service use of the coated article, the app ied layers becoming denser and more resistant to the infiltration of air with the passage of time.

Under certain conditions, for example when the coated article is used at lower temperatures, it may be desirable to bake the coated part or structure in order to expedite surface alloying of the coating layers with each other and of the inner layer with the foundation metal. Such baking step may take place after applying each layer, but ordinarily one baking of the article with its composite coating thereon will suffice. For best results the time and temperature conditions of baking a particular oxidizable metal having a particular coating should be determined experimentally. Generally speaking, the temperature of baking may vary between 500 and 1000 C. and the time from 15 minutes to four hours or longer. Baking may be carried out in an oxidizing or a reducing atmosphere, but somewhat better results are obtained under reducing conditions. 'We prefer to use an atmosphere of pure, dry hydrogen.

Flat foundation metal, either after applying the initial coating of aluminum or after applying the sealing or the final metallic coating layers, may be hot or cold rolled or swaged to densify the coating layer or layers. This serves to.increase the imperviousness of the coating to penetration by air. The rolled or swaged article may or may not be baked as above-described prior to completion of the metallizing operation or before service use of the calorized article.

When repairing damaged calorized surfaces, the surface material should be ground away with a coarse wheel or the like to provide a roughened surface before applying the metallic coatings.

Steel boiler tubes protectively covered with a composite, sprayed (Schoop 'method) coating of aluminum, tin and aluminum have been tested under practical operating conditions for over a year without noticeable oxidation of the steel surface.

The term "calorized is used herein in the broad sence of an oxidizable metal treated to render surfaces thereof substantially inoxidizable when the article is heated at an elevated temperature in an oxidizing atmosphere.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The process of treating an oxidizable, solid foundation metal to render it inoxidizable at a high temperature which comprises applying to a clean surface of the metal a coating of aluminum, applying over the aluminum a coating of a different metal having a melting point below approximately 660 C. and capable of alloying with the aluminum to form a composite layer that is substantially impervious to air at an elevated temperature, and causing the said aluminum to alloy at least in part with each of the underlying and overlying metals.

2. The process of protecting a ferrous metal from oxidation which comprises spraying upon the clean surface of such metal a coating of aluminum, spraying over the aluminum a coating of a different metal having a melting point below approximately 660 C. and capable of alloying with the aluminum coating to form a composite layer that is substantially impervious to air at an elevated temperature, and causing the said aluminum to alloy at least in part with each of the said underlying and overlying metals.

3. The process of protecting steel from oxidation at high temperature which comprises spraying upon the clean surface of the steel 9. coating of aluminum, spraying over the aluminum a coating of tin, spraying over the tin a second coating of aluminum, causing the first aluminum layer to alloy at least in part with the underlying steel and causing the tin layer to alloy at least in part with each of the underlying and overlying aluminum layers.

4. A metal product comprising a solid, oxidizable foundation metal and a composite metallic covering thereon which renders said foundation metal inoxidizable at a high temperature, said covering comprising a coating of aluminum at least in part alloyed with the said foundation metal, and over said aluminum coating and at least in part alloyed therewith a coating of a different metal having a melting point below approximately 660 C. and being capable of alloying with the aluminum to form a composite mass that is substantially impervious to air at an elevated temperature.

5. As a new article of manufacture, a ferrous metal and a composite metallic covering thereon which renders said ferrous metal inoxidizable at a high temperature, said covering comprising a coating of aluminum at least in part alloyed with the said ferrous metal, and over said aluminum coating and at least in part alloyed therewith a coating of a different metal having a melting point below approximately 660 C. and being capable of alloying with the aluminum to form a composite mass that is substantially impervious to air at an elevated temperature.

6. A metal product comprising steel and a composite metallic covering thereon which renders the steel 'inoxidizable at a high temperature, said covering comprising a coating of aluminum at least in part alloyed with the steel, a coating of tin directly upon the aluminum coating and at least in part alloyed therewith, and a second coating of aluminum directly upon 3 1 5111 coating and at least in part alloyed there- 7. The process of treating a solid, oxidizable foundation metal to render it inoxidizable at a high temperature which comprises applying to a clean surface of the metal a coating of aluminum, applying over the aluminum a second coating of a different metal having a melting point below approximately 660 C. and which is capable of forming an alloy with the aluminum, applying over said second coating another coating of aluminum, causing the first aluminum coating to alloy at least in part with the underlying foundation metal, and causing the said second coating of metal having a melting point below approximately 660 C. to alloy at least in part with each of the underlying and overlying aluminum coatings.

8. The process of protecting a ferrous metal from oxidation which comprises spraying upon the clean surface of such metal a coating of aluminum, spraying over the aluminum a second coating of a different metal having a melting point below approximately 660 C. and which is capable of forming an alloy with the aluminum, spraying over said second coating another coating of aluminum, causing the first aluminum coating to alloy at least in part with the underlying ferrous metal, and causing the second coating of metal having a melting point below approximately 660 C. to alloy at least in part with each of the underlying and overlying aluminum coatings.

9. The process of rendering a solid, oxidizable foundation metal resistant to oxidation at a high temperature which comprises applying to a clean surface of the metal a coating of aluminum, applying over the aluminum a coating of tin, applying over the tin coating a second coating of aluminum, causing the first aluminum coating to alloy at least in part with the underlying foundation metal, and causing the tin coating to alloy at least in part with each of the underlying and overlying aluminum coatings.

FRANK B. QUINLAN. LLOYD P. GRQBEL. 

